Project Summary/Abstract Trillions of microbes reside in and on our body's barrier surfaces. To maintain health our immune system must establish tolerance to these commensal microbes but also preserve the ability to mount protective responses against infectious threats. We have found that adaptive immune tolerance is established to skin commensal bacteria but not skin pathogens and that these divergent responses are established upon first encounter with these microbes early in life. The overarching goal of this proposal is to elucidate the host-directed and microbe-directed mechanisms that support adaptive immune tolerance to commensal microbes while permitting protective immunity to pathogens, using skin as the tissue of focus. To achieve this we will employ cutting-edge immunological and microbiological techniques, including in vivo tools to dissect the antigen-specific response to skin bacteria, a new CRISPRi system to genetically modify skin bacteria and identify key microbial molecules, and a novel pre-clinical platform to examine the impact of microbial products on a human inflammatory skin disease. Ultimately, this high-risk high-reward proposal seeks to inform our understanding of fundamental mechanisms that shape our early `decisions' about tolerance vs. immunity to foreign antigens and identify new therapeutic approaches to modulate these responses for clinical benefit.